The growth of diamond thin films from the vapor phase have been tried according to several kinds of methods, for example, thermal or plasma chemical vapor deposition (CVD) [S. Matsumoto, Y. Sato, M. Kamo and N, Setaka, Jpn. J. Appl. Phys. 21, L 183 (1962); S. Matsumoto, Y. Sato, M. Tsutsumi and N. Setaka, J. Mater. Sci. 17, 3106 (1982) 29; B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, J. Cryst. Growth 52, 219 (1981)]; ion beam techniques [S. Aisenberg and R. Chabot, J. Appl Phys. 42, 2953 (1971); E. G. Spencer, P. H. Schmidt, D. C. Joy and F. J. Sansalone, Appl Phys. Lett. 29, 118 (1976); C. Weissmantel, K. Bewilogua, D. Dietrick, H. J. Erler, H. J. Hinneberg, S. Klose, W. Nowick and G. Reisse, Thin Solid Films 72, 19 (1980)] and plasma induced deposition [T. Mohri and Y. Namba, J. Appl. Phys. 55, 3276 (1984)]. The films obtained by these methods are usually called diamondlike carbon films, since they have some properties similar to those of natural diamond. From the identification of the structure by electron diffraction, however, most of these films are difficult to regard as diamond because of their amorphous structures, lack of diffraction lines which should appear, and the appearances of diffraction lines from the structure other than diamond. It is known that the diamond particles and films with good crystallinity are obtained by thermal and plasma CVD. Even in these two methods, it is hard to form a uniform film on any substrates except for diamond substrates, and the growth rate of the film is relatively low.
However, the present inventors found a surprising fact that a diamond thin film was formed when CVD was performed under an electron bombardment, thus resulting in the present invention. A part of the process of the present invention was contributed and accepted for its originality (Appl. Phys. Lett. 46(2), 146 to 147, 15 Jan. 1985). In said scientific journal, the present inventors have termed the process "EACVD". Namely, EACVD is the process in which a diamond crystal nucleus is caused to form and grow to a thin film on a heated plate substrate under an electron bombardment in an atmosphere of a mixed gas of a hydrogen and a hydrocarbon in a reduced pressure.
Accordingly, one object of the invention is to provide a process for preparing a diamond thin film by EACVD.
Another object of the invention is to provide a new material in the fields of an abrasive material, a hardwearing material, an electrical insulating material for high cooling utilizing the diamond characteristics of high thermal conductivity, ultra-high hardness, high electrical insulation properties.
Further, the present inventors have achieved the process for preparing a Boron doped p-type diamond semiconductor in a thin film state after many trials and researches for EACVD using a mixed gas of a hydrogen, a hydrocarbon and a diborane.
Accordingly, a further object of the present invention is to provide industrially a p-type diamond semiconductor in a thin film state.